MicroRNAs are small non-coding RNAs that have recently become recognized as important molecular regulators in human cancer. However, despite the knowledge that microRNA expression is frequently deregulated in acute myeloid leukemia (AML), the role of microRNAs in the leukemic initiation and maintenance remains poorly understood, the understanding of which is our long-term goal. Following our previous work in trying to understand the role of microRNAs in hematopoiesis and in human cancers, here, we aim to understand the role of a specific microRNA family, the miR-125 family of microRNAs, in AML. We found that miR-125 family microRNAs display in vitro property associated with transformation in hematopoietic cells. Moreover, we have accumulated further evidence suggesting a role of these microRNAs in the leukemogenesis of AML. Through extensive microRNA expression profiling in human AMLs, we found a subgroup of AMLs display greatly elevated miR-125a or miR-125b levels. We also found that miR-125a expression is high in self-renewing hematopoietic stem cells (HSCs). Forced expression of miR-125a positively modifies stem cell activity and antagonizes the apoptotic pathway. In addition to our own work, it has been reported that miR-125b-1 is translocated in a subgroup of human AMLs and myelodysplastic syndromes. The major goal of this proposal is to determine the roles of miR-125 family microRNAs in leukemic initiation and maintenance, through developing and analyzing mouse models and human cells. In Specific Aim 1, we will use several mouse models to determine the role of miR- 125 family microRNAs in the initiation of pre-leukemic state and AML. In Specific Aim 2, we propose to identify the cellular and molecular mechanisms by which miR-125 microRNAs deregulate normal hematopoiesis. In Specific Aim 3, we will determine the role of miR-125 microRNAs in the maintenance of pre-leukemic and leukemic conditions in mouse models and in human cells. Collectively, the proposed research will advance our understanding of the role of microRNAs in leukemic initiation and maintenance by specifically addressing the role of miR- 125 family microRNAs in acute myeloid leukemia. Success of this work will lead to a refined appreciation of a group of prevalent fatal diseases on the basis of small RNAs, and could establish immediate targets for therapeutic intervention.